1. Field of the Invention
The present invention relates to an image processing apparatus for image reduction processing and a control method thereof.
2. Description of the Related Art
The number of pixels in image sensors such as CCD image sensors and CMOS image sensors incorporated in capturing apparatuses such as digital cameras is increasing, and image sensors having more than ten megapixels are not rare any more. An increase in the number of pixels leads to an increased amount of image data, and therefore there are cases where an image (original image) captured at the maximum number of pixels is subjected to image reduction processing according to the application of the image.
Generally, image reduction is implemented by pixel thinning or interpolation, and in order to suppress a situation in which high frequency components appear as aliasing noise due to the pixel thinning or interpolation, preprocessing is performed to remove the high frequency components from the original image using a band-limiting filter (low-pass filter).
Generally, if a low-pass filter that can limit the original image to a lower frequency band can be used, a reduced image reduced at a higher reduction ratio and having high image quality can be obtained while suppressing the influence of aliasing noise.
Japanese Patent Laid-Open No. 2001-036812 discloses a configuration that implements a low-pass filter using two line memories. However, in order to obtain an image having high image quality and a high reduction ratio (for example, a vertical reduction ratio of 1/5 or 1/10), a prefilter having a large number of taps is required.
In addition, Japanese Patent Laid-Open No. 2007-067917 discloses a configuration that suppresses the size of image processing circuitry and the image processing load by converting input image data to intermediate image data. However, conversion of input image data to intermediate image data each time an image having a different reduction ratio is generated requires a large number of memory reading and writing operations.
In order to implement a low-pass filter for limiting an image to a low frequency band, a large number of line memories are required, which results in not only an increase in circuitry size but also an increase in cost.
On the other hand, in the case where a low-pass filter circuit having a reduced number of line memories and a reduced circuitry size and cost is used for image reduction processing, a problem arises in that band limitation (or in other words, suppression of aliasing noise) will be insufficient if the reduction ratio is high, resulting in a reduced image having reduced image quality.